Projector system

ABSTRACT

A projector system comprises an illumination-optical system ( 1 ), a colorseparating system ( 4 ) which separates light coming from the illumination-optical system ( 1 ) into at least three beams of colored light (R,G,B), the system further comprising, for each beam of colored light, a modulation device ( 16, 18, 19 ) for modulating the beam of colored light, and a color combiner ( 20 ) which combines the modulated color beams. The projector system comprises a polarization conversion system (PCS) for at least one colored beam and no polarization conversion system for at least one of the other colored beams.

FIELD OF THE INVENTION

The invention relates to a projector system comprising anillumination-optical system, a polarization conversion system, acolor-separating system which separates light coming from theillumination-optical system into at least three beams of colored light,the system further comprising, for each beam of colored light, amodulation device for modulating the beam of colored light, and a colorcombiner which combines the modulated color beams.

BACKGROUND OF THE INVENTION

Such a device is known from European patent application no 1 071 292.

A projector usually comprises an illumination-optical system, acolor-separating system and modulation devices, such as liquid crystalpanels for modulating the colored light beams. The colored modulatedlight beams are then combined and projected onto a screen by aprojection lens or lens system. If the modulators add image informationto the light beam by means of polarization modulation, the modulator,such as a liquid crystalline display panel, is generally situatedbetween two polarizers. If unpolarized light is incident on the displaypanel, substantially half of it will be absorbed by the first polarizerand thus will be lost by the formation of the image. This absorptionalso gives rise to heating of the polarizer and the display panel. Toalleviate this problem, the known device comprises a polarizationconversion system (PCS). A polarization conversion system converts thelight emitted by the light source, which is not polarized into apolarized light beam preferably with a minimum loss of light. In orderto do so, conventional polarization conversion systems have two arraysof lenses and a polarized light-generating system, the polarizationconversion system comprising a shading plate, a polarizing beam splitterarray and a selective retardation plate. The lens arrays are arranged insuch a way that the light from the light source is split into a numberof beams and that said beams enter only at the transparent parts of theshading plate. The polarizing beam splitter separates the entering beamsinto s-polarized beams and p-polarized beams. One type of these beams issent to a polarization-altering element, such as a selective retardationplate wherein the polarization is changed from s to p or from p to s.The end result is that the light exiting the polarization conversionsystem is polarized in one direction, the s or p direction. This lightis then sent to the color-separating system, modulated by the respectivecolor modulation devices, recombined in the color combiner and projectedon a screen.

Although the known system works satisfactorily and offers advantages, anumber of problems remain. The polarization conversion system is a veryexpensive system, and it is difficult to obtain and maintain a highquality for this system.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to alleviate or at least reduce one ormore of the above-mentioned problems.

To this end, the projection system according to the invention ischaracterized in that the projector system comprises a polarizationconversion system for at least one colored beam and no polarizationconversion system for at least one of the other colored beams.

The projection system according to the invention thus has a polarizationconversion system for one of the colors, i.e. at a position at which thecolors are separated. The other color beams are thus not polarizedbefore they enter the modulation devices.

Prima facie, one might think that this is a step backwards from theknown system.

However, the inventors have realized the following.

Systems based on light modulation use projection lamps that emitunbalanced red, green and blue colors. High brightness, costs and aproper color reproduction are the most important aspects of thesesystems. For proper color reproduction, which is of special importancefor projecting photo and video images, up to 40% of two of the colors(usually green and blue) is actually thrown away and cannot be used athigh intensity, because it would not give a proper color rendition,especially not a proper white color balance. The PCS in known projectorsystems has to work for all colors, which requires a costly system and,in reality, some compromise will have to be made wherein a less thanperfect polarization conversion is obtained for all colors so as to getan optimum for all colors combined. Even with a PCS, some percentage ofthe light will thus be lost in known systems, because the PCS is lessthan perfect and the PCS itself (due to e.g. reflections on surfaces)may be a cause of loss of intensity. All in all, not using the PCS fortwo colors does not seriously affect the loss of light for these colors.In fact, removing the PCS from these light paths reduces the number ofelements in these light paths, which reduces the complexity of theoptical system and may even lead to an improvement for these colors. APCS is positioned in the light path of the third color, usually red,i.e. after the light beams have been separated. This PCS deals with onecolor only, i.e. with a much smaller wavelength range than the PCS inthe known system. Almost all constituent elements (splitters,retardation plates, etc.) of the PCS have optical properties that arewavelength-dependent, which makes it difficult and costly to provide aPCS system operation for all colors. However, in the system according tothe invention, the PCS system deals with one color only, and this allowsa reduction in costs and yet an improvement in performance. Forinstance, anti-reflective layers which reduce reflection on surfaces forall visible wavelengths are usually much more complex than for red only.

The overall system thus does not suffer from a significant loss ofquality for the two colors paths in which there is no PCS in theprojector system according to the invention, and there is some increaseof quality for the color path in which there is a PCS in the system inthe invention, while a significant cost reduction is obtainable.

The invention thus offers the possibility of a cost reduction without anappreciable reduction of image quality and without requiring a majorchange of design (although some design changes are needed).

The projection system preferably comprises, for at least one of thecolor beams, a relay system between the color-separating system and thecolor combiner, and a polarization conversion system is positioned inthe relay system.

The PCS is preferably positioned in the relay system. In the relaysystem, the beam path is lengthened to provide room for positioning aPCS without the need to considerably change the general layout of theprojector system or of the color-separating system in particular.

It is noted that it is known per se to provide a relay system in or forthe color-separating system. “Between the color-separating system andthe color combiner” is meant to indicate the positioning of the relaysystem seen along the beam path.

In a preferred embodiment, the PCS is positioned in the red beam path,and, in operation, the relay system preferably relays a red beam.

Most lamps have an unbalance in the light emission, such that blue andgreen light have to be reduced for a proper white balance. In suchcircumstances, it is advantageous to provide the PCS in the red beampath. Within the broader concept of the invention, the PCS may beprovided for colors other than red, provided that the color for whichthe PCS is provided is a color that is relatively lacking for a properwhite balance from the light emitted by the illumination system. To putit differently, in such circumstances, the other colors are relativelytoo bright for a proper white balance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a sectional view of a known projector system.

FIGS. 2, 3(A) and 3(B) illustrate schematically one example of a PCS.

FIG. 4 illustrates a projector system according to the invention.

FIGS. 5 and 6 illustrate details of the system shown in FIG. 4.

The cooperation of various parts of the system shown is detailed in FIG.7.

The Figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the Figures.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a projector system as known from the prior art. Theprojector system comprises an illumination-optical system 1, comprisinga lamp and a parabolic projector 2. The light emitted by the lamp passesa polarization conversion system (PCS) 3. This system converts theunpolarized light emitted by the lamp into polarized light. One way ofdoing this is explained in FIG. 2. The light is split into three coloredlight beams in a color-separating system 4 comprising a number of(semi-transparent) mirrors 5, 7 and a folding mirror 6. In one of thelight paths in the known system, the blue light is relayed via a relaysystem 8 having a number of mirrors 9, 10 and a number of relay lenses11, 12, 13. Via lenses 13, 15 and 17, the three light beams are incidenton modulators 16, 18 and 19 by which the colored light beams aremodulated. The light beams are recombined in color combiner 14 andprojected on a screen 22 via projector lens system 21.

FIG. 2 illustrates PCS 3 as described for the known projector system inEP 1 071 292. In general, in a polarization conversion system, theoriginal light beam having different directions of polarization isseparated by a polarization-sensitive beam splitter into two sub-beamshaving different directions of polarization that are perpendicular toeach other. In a polarization conversion means, the direction ofpolarization of one of these beams is converted into the polarizationdirection of the other one of said beams, and the converted sub-beam issubsequently combined with the unconverted sub-beam to provide, incombination, a polarized light beam.

FIGS. 2,3(A) and 3(B) illustrate schematically one example of a PCS. Twoarrays 21, 23 of lenses 22, 24 are arranged in front of the actual PCS25. These lenses split the incoming, nearly parallel light beam comingfrom the lamp and reflector into a number of partial light beams. Thepartial light beams enter the PCS 25, which comprises a shading plate 31having light-blocking areas 32 (which could be reflective) andtransmissive areas 33. The light is concentrated and passes thetransmissive areas 33 entering a polarization-sensitive beam splitter 34with transmissive members each having a roughly parallelogram shape.Each member has a surface 35 that passes light having one polarizationdirection (in this example, the p-polarization) while reflecting lightwith the other polarization (in this example, s-polarisation), and thereflected light is again reflected on surface 36. The s and p-polarizedlight beams enter a polarization conversion means 37 having areas 38 and39 in which one of these areas changes the polarization direction of thelight by 90 degrees. In this example, the p-polarized light beams areconverted into s-polarized light beams. The net result is that theincoming unpolarized (s+p) light is converted in PCS 25 into polarized(in this example, s-polarized) light. Quarter-wave films may be used forsuch a conversion.

The modulators 16, 18 and 19 usually comprise polarizers or operate onlyon polarized light. Light with the “wrong” polarization is absorbedbefore or in the modulators and is thus lost, leading only to anincrease of heat in the projector system. The provision of the PCSsystem therefore offers advantages.

However, systems based on light modulation use projection lamps thatemit unbalanced red, green and blue colors. High brightness, costs and aproper color reproduction are the most important aspects of thesesystems. For proper color reproduction, up to 40% of two of the colors(usually green and blue) is actually thrown away and cannot be used ator near full intensity, because it would not give a proper colorrendition, especially not a proper white color balance. This leads to aconsiderable heat input. To put it simply, there is far too muchintensity in two of the color beams, and not enough in the third, andthe surplus in the first two is thrown away to obtain proper whitecolors. Any white unbalance will lead to this situation, although theamount of unbalance and the actual colors that are too bright or tooweak, and the extent of brightness or weakness of these colors maydiffer in dependence on the lamp used. In most systems, the surplus ofblue and green is some 40%, as described. This surplus is comparable towhat is gained by using the PCS system for these colors. The PCS systemitself is a costly system and complicated, in particular because it hasto function over all wavelengths. The inventors have realized that thecost can be reduced by putting a PCS only in one of the color lightpaths, i.e. after splitting the light beams into color light beams.Putting the PCS in the “weakest” light beam enables the PCS to be madesimpler because only a limited wavelength range is of importance, thussaving costs, while yet functioning better. Many or all of the elementsused in the PCS are polarization-dependent but usually alsowavelength-dependent. A quarter-wave plate that needs to work for onlyone color is much easier to construct than a quarter-wave plate thatneeds to operate for the full white light range. A quarter-wave platethat needs to work for one color can be made from a single, uniaxialfilm, while a wideband quarter-wave plate that needs to work in the fullvisible center is made of a stack of 3 (or even more) uniaxial films.Similar anti-reflex coatings for a wide spectral range require a stackwith a larger number of layers than anti-reflex coatings that need tohave low reflections at only a single color. Restricting the PCS to onecolor only removes many of these problems and therefore offers thepossibility for a better PCS to be made for less money. For the twoother colors, the optical system is simplified, which also offersadvantages because the more elements an optical system contains, themore losses may generally occur and the more stringent requirements willbe imposed, such as for alignment of optical elements. It is true that50% of the light in these two light channels will have to be dumped, butin the existing system a small percentage is always lost because of thepresence and inherent imperfection of the PCS and, in practice, some 40%is lost due to the unbalance in the light emitted by the lamp. The netresult for these two colors is thus small. The net result for the colorin which the PCS is positioned is positive because a PCS can be madewhich is better tuned to the color of the light

Most lamps have an unbalance in the light emission, such that blue andgreen light have to be reduced for a proper white balance. In suchcircumstances, it is advantageous to provide the PCS in the red beampath. Within the broader concept of the invention, the PCS may beprovided for colors other than red, provided that the color for whichthe PCS is provided is a color that is relatively lacking for a properwhite balance from the light emitted by the illumination system. To putit differently, in such circumstances, the other colors are relativelytoo bright for a proper white balance. It is remarked that, in mostcurrent systems, light is separated into three colored beams, which aremodulated and then recombined. In systems where the light would beseparated into more than three colors (e.g. four), the invention is alsoapplicable.

FIG. 4 illustrates a projector system according to the invention. FIGS.5 and 6 illustrate details of the system shown in FIG. 4.

Compared to the known system, the PCS is removed from the part of theprojector system before the color separation system, and the red beam isrelayed, and a PCS 41 is provided between lenses 42 and 43 in the relaysystem. FIG. 4 shows the system schematically.

FIGS. 5 and 6 illustrate some details of the system schematically shownin FIG. 4.

FIG. 5 illustrates the light illumination system. It comprises a lampand reflector 1, a UV filter 51, and two integrators 52 and 54, eachcomprising a number of lenses 53, 55, positioned in such a way that theparallel beam entering the first integrator is split into a number ofparallel beams exiting the second integrator. For the side beams, thisis schematically indicated in the Figure. The light is reflected by amirror 57, which in this embodiment doubles in function as a IR filter,relayed by lens 58 to a first dichroic color filter 59 (comparable tocolor filter 5 in FIG. 4). It is remarked that FIG. 4 shows the designschematically and in a condensed form with some details not being shownin FIG. 4 so as to make it possible to see the general design of thedevice. In particular, for simplicity, the mirror 57, and relay lens 58(as well as the angle in the light path due to the provision of themirror 57) are not shown in FIG. 4 but in FIG. 5. The actual light pathbetween the light illumination system and the color separation systemdoes not limit the scope of the invention. The blue beam is transmittedby mirror 59, while the red and green light are reflected upwards. Alllight at this stage is unpolarized, i.e. it comprises s and ppolarization.

FIG. 6 illustrates the next part of the projector system. The elements57, 58 and 59 are the same as in FIG. 5. The R and G beams are split bydichroic mirror 62. The Blue (B) and Green (G) are modulated bymodulation devices 61 and 63. Field lenses are arranged in front of themodulation devices (usually LCD devices). As already discussed, giventhe fact that the light incident on the B and G modulation devices 61,63 is unpolarized and the modulation devices conventionally comprisepolarizers, 50% of the G and B light will not pass the modulators, butwill be lost. However, as already explained, due to the color unbalanceof the illumination device, some 40% of the light would have to bestopped (in the modulation devices) anyway.

The red beam is relayed via lens 64 and mirrors 66 and 67 to modulationdevices 68 via field lens 69. In the relay system, a PCS 41 is arrangedbetween lenses 42 and 43. The image of the integrators 52, 54 is formedbetween lenses 42 and 43, thus forming a pattern of bright spots, whichin fact form the image of the bright spots at integrator 54, which spotsare schematically indicated in FIG. 5 by the points where the linesindicating the light beams cross one another. There are dark spacesbetween these bright spots. In this example, a reflective polarizer 70is positioned in front of the modulation device.

The cooperation of the various parts in the last part of the system isshown in detail in FIG. 7. Arranging a mirror pattern 71 between lenses42 and 43 at or near the location of these bright spots allows the lightto pass the mirror pattern, while the light that has passed the mirrorpattern will be incident on reflective polarizer 70 in front ofmodulation device 68. The “right” polarization will pass and bemodulated, whereas the “wrong” polarization will be reflected. Thevarious lenses and other optical elements are aligned in such a way thatthe reflected polarization mode generates a second image of theintegrator plate at substantially (preferably exactly) the same planebetween the lenses 42 and 43, i.e. a pattern of bright spots with darkspaces in between, but slightly shifted in a direction in the plane,such that the light is re-reflected by the mirror pattern. The mirrorpattern is provided with a quarter-wave film, to rotate the polarizationof the bounced light This light will pass the reflective polarizer.Thus, a PCS system is provided in the relay system. This PCS comprisesthe mirror pattern 71 which is positioned in such a way that the brightspots coincide with the holes in the mirror pattern and thus passthrough the quarter-wave film 72 at the back of the mirror pattern andthe reflective polarizer 70 in front of the modulator. Such anarrangement with a patterned mirror plate, provided with a quarter-wavefilm and a reflective polarizer is a preferred embodiment, as thearrangement of a patterned mirror plate and a quarter-wave film is muchsimpler and thus less costly, as can be seen when compared to the PCS ofFIGS. 3A and 3B. The provision of the reflective polarizer does not addsubstantial costs.

While the invention has been described in connection with preferredembodiments, it will be understood that modifications thereof within theprinciples outlined above will be evident to those skilled in the art,and thus the invention is not limited to the preferred embodiments butis intended to encompass such modifications. The invention resides ineach and every novel characteristic feature and each and everycombination of characteristic features. Reference numerals in the claimsdo not limit their protective scope. Use of the verb “to comprise” andits conjugations does not exclude the presence of elements other thanthose stated in the claims. Use of the article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

1. A projector system comprising an illumination-optical system (1), acolor-separating system (4) which separates light coming from theillumination-optical system (1) into at least three beams of coloredlight (R,G,B), the system further comprising, for each beam of coloredlight, a modulation device (16, 18, 19) for modulating the beam ofcolored light, and a color combiner (20) which combines the modulatedcolor beams, characterized in that the projector system comprises apolarization conversion system (PCS) for at least one colored beam andno polarization conversion system for at least one of the other coloredbeams.
 2. A projector system as claimed in claim 1, characterized inthat that the projector system comprises, for at least one of the colorbeams (R), a relay system (8) between the color-separating system (4)and the color combiner (20), and in that a polarization conversionsystem (PCS) is positioned in the relay system.
 3. A projector system asclaimed in claim 1, characterized in that the PCS is positioned in thered beam path (R).
 4. A projector system as claimed in claim 1,characterized in that the PCS comprises a patterned mirror plate (71)provided with a quarter-wave film (72) and a reflective polarizer (70).5. A projector system as claimed in claim 1, characterized in that noPCS is provided for any of the other colors.